One of the recent trends in the communications art is the development of digital communication systems, capable of transmitting and receiving data and voice information utilizing digital techniques. In such systems, a voice pattern, for example, is first digitized to form a digital data stream and then transmitted over a transmission path to a receiver. The receiver, in turn, decodes the digital signal and converts it back into the original voice pattern.
All digital communication systems require some degree of synchronization in order to properly detect an incoming carrier signal, recapture the transmitted digital information, and insure the quality of the reconstructed speech. In this context, synchronization pertains to a relationship between the transmitter and the receiver wherein they maintain concurrent frequency and phase. It will be appreciated, however, that most digital communication systems employ several levels of synchronization in order to achieve these goals. Some examples are: phase, symbol, frame, and network synchronization.
Phase synchronization occurs when the receiver establishes a phase concurrence between an incoming carrier sinusoid and a receiver reference signal such that the incoming sinusoid and the reference pass through zero simultaneously. This condition is known as phase lock, and will enable the receiver to successfully detect the presence of a carrier signal.
Symbol (bit) synchronization is achieved when the receiver produces a square wave that transitions through zero simultaneously with the incoming signal's transitions between symbols (bits). This synchronization is required in order to establish the proper symbol interval, thereby enabling the receiver to make accurate symbol decisions.
Frame synchronization is required when the transmitted data is organized into blocks, packets, or messages of a uniform number of symbols. Frame synchronization is somewhat equivalent to generating a square wave having zero crossings that coincide with the transitions from one frame to the next.
Network synchronization is achieved when a phase concurrence between the transmitter master clock and the receiver master clock is established. This synchronization is employed in order to assure the timing relationship between the transmitter and the receiver does not drift. Timing drifts cause both bit slippage and bit errors, which drastically reduce the quality of digitized speech during the reconstruction of a voice pattern.
It will be appreciated by those skilled in the art that each level of synchronization utilized in a digital communication system implies additional cost. Aside from the obvious expense associated with the increased hardware and software needs, hidden costs are to be suffered as well. One cost is the additional energy expended by the transmitter producing signals used by the receiver in an effort to match operation. Another penalty is that the increased time required to achieve synchronization delays the commencement of communication. Moreover, the increased demand synchronization places on processor time reduces overall system throughput.
Currently, the entire Public Switching Telephone Network (PSTN) operates from a centrally located master clock source or universal time clock that gets distributed to all user centers nationwide. The universal time clock in the United States is generated by the U.S. National Bureau of Standards for the very purpose of providing worldwide synchronization. Due to the added expense associated with a highly synchronized operation, it is proving increasingly impractical for some digital communication systems to employ high levels of synchronization in order to interface with the PSTN. One such example is an in-building radio telephone communication system.
It would therefore be extremely advantageous to provide an alternative approach to maintaining the quality of digitized speech without the need for a highly synchronized systems.